CO2 Conversion on N-Doped Carbon Catalysts via Thermo- and Electrocatalysis: Role of C–NOx Moieties

N-doped carbon (N–C) materials are increasingly popular in different electrochemical and catalytic applications. Due to the structural and stoichiometric diversity of these materials, however, the role of different functional moieties is still controversial. We have synthesized a set of N–C catalysts, with identical morphologies (∼27 nm pore size). By systematically changing the precursors, we have varied the amount and chemical nature of N-functions on the catalyst surface. The CO2 reduction (CO2R) properties of these catalysts were tested in both electrochemical (EC) and thermal catalytic (TC) experiments (i.e., CO2 + H2 reaction). CO was the major CO2R product in all cases, while CH4 appeared as a minor product. Importantly, the CO2R activity changed with the chemical composition, and the activity trend was similar in the EC and TC scenarios. The activity was correlated with the amount of different N-functions, and a correlation was found for the −NOx species. Interestingly, the amount of this species decreased radically during EC CO2R, which was coupled with the performance decrease. The observations were rationalized by the adsorption/desorption properties of the samples, while theoretical insights indicated a similarity between the EC and TC paths.

. to determine the onset potentials of the reduction reaction.
. CO partial current densities on the studied N-C catalysts, normalized by the roughness factor of the electrodes (1.00 mg cm -2 loadings). Measurements were performed in a CO 2 -saturated KHCO 3 solution. Lines serve as a guide for the eye. Amine N-content / at. %     . Electronic adsorption energies of CO 2 (left) and "one step" CO 2 +1/2H 2 adsorption (right) over all symmetrically non-equivalent sites of the examined defects (top inset). No local minima were found corresponding to chemisorbed CO 2 molecules bound via a direct C(CO 2 )-N(defect) bond (left), resulting in vdW adsorbed configurations. However, stable COOH motifs were found for several defect motifs (motifs), most notably on the pyrrolic N atoms (as in the NO+N6+2N5 defect).  Table S1. Roughness factors determined from the double layer capacitance values (cyclic voltammetry) and BET surface areas calculated from the N2 asorption / desorption isotherms of the studies N-C samples.    Table S5. Intensity ratios of the D and G bands in the Raman spectra of the studied catalysts.

Theoretical and computational details
For the calculation of the Gibbs energies for the thermal path, we have used While the pH is not present in the thermal path, the term 10 k b T ln(Q) has been estimated for each reaction with The translational and rotational contribution to the enthalpy was approximated as 3/2 k b T The vibrational contribution of the entropy was approximated as Where R is the is ideal gas constant, h is the Planck constant, is the frequency, k b is the Boltzmann constant. For all tables we named the surfaces with the convention clean is the basic 4N and a O cavity, hrol is the cavity with a pyrrolic H, hdin is the cavity with a H pyridinic and hh with both pyrrolic and pyridinc. All applied voltage are vs RHE. All numbers in the following tables are given in eV.
The following CO path without allowing *H transfer was investigated: Surface ΔG(*COOH) (eV) The path without allowing H transfer from the cavities all have an overpotential > 0 and are therefore not thermodynamically meaningful. The optimal path presented in the paper follow the reaction presented in Table 1 of the manuscript. For the electrochemical path:
We see that the step 6 is blocking the reaction at 298.15 K.